The present invention relates to a polymer which contains at least one structural unit which has electron-transport properties, to processes for the preparation thereof, to mixtures (blends), solutions and formulations which comprise these polymers. Furthermore, the present invention relates to the use of these polymers in electronic devices, in particular in organic electro-luminescent devices, so-called OLEDs (OLED=organic light emitting diodes), and to these organic electroluminescent devices themselves. The polymers according to the invention exhibit improved efficiency and a long lifetime, in particular on use in OLEDs.
Polymers for opto-electronic applications are preferably either conjugated or partially conjugated main-chain polymers, in which the polymer back-bone itself plays an important role with respect to the opto-electronic properties, side-chain polymers, whose functionality is brought about by transport units and/or emitters which is chemically bonded to the backbone, or neutral polymers, which are responsible only for the film-forming properties (known of organic photoreceptors, in which the hole-transport materials are typically dissolved in polycarbonate).
Polymers have already been investigated intensively for a long time as highly promising materials in OLEDs. OLEDs which comprise polymers as organic materials are frequently also known as PLEDs (PLED=polymeric light emitting diodes). Their simple preparation promises inexpensive production of corresponding light-emitting devices.
Since PLEDs usually only consist of one light-emitting layer, polymers are required which are able as far as possible to combine all functions (charge injection, charge transport, recombination) of an OLED in themselves. In order to meet these requirements, different monomers which take on the corresponding functions are employed during the polymerisation. Thus, it is generally necessary, for the generation of all three emission colours, to copolymerise certain comonomers into the corresponding polymers (cf., for example, WO 00/46321 A1, WO 03/020790 A2 and WO 02/077060 A1). Thus, it is possible, for example starting from a blue-emitting base polymer (“backbone”), to generate the other two primary colours red and green.
Various classes of material, such as, for example, poly-para-phenylenes (PPPs), have already been proposed or developed as polymers for full-colour display elements (full-colour displays). Thus, for example, polyfluorene, polyspirobifluorene, polyphenanthrene, polydihydrophenanthrene and polyindenofluorene derivatives come into consideration. Polymers which contain a combination of the said structural elements have also already been proposed.
The most important criteria of an OLED are efficiency, colour and lifetime. These properties are determined not only by the emitter(s) used, but also are crucially dependent on how well the charge transport in the OLED can take place. It is known that both electron transport and also hole transport in a PLED take place in the layer of the light-emitting polymer (LEP layer). In order to improve the efficiency and lifetime of OLEDS, it is necessary to simplify the injection or transport of the charges into the LEP layer. In order to improve so-called hole injection or hole transport, a number of novel monomers based on triarylamine derivatives have been proposed. These have enabled the efficiency and lifetime of such devices to be improved. However, virtually no stable electron-transport units are described in the literature.
For this reason, predominantly improvements in the electron-transport units compared with the materials known from the prior art are required.
Since PLEDs are usually composed of only one layer, the units which facilitate charge transport must be incorporated directly into the polymer through polymerisable monomers. Thus, oxadiazole, which is known as electron-transport unit, has already been incorporated into conjugated and non-conjugated polymers as main-chain component and side-chain component. The efficiency of devices comprising such polymers was fairly satisfactory, but the lifetime was inadequate for use in OLEDs. Likewise, compounds containing a plurality of keto groups have also been used. However, corresponding polymers have the disadvantage that, owing to the energy gap of such molecules, they are not compatible with units which emit in the blue spectral region.
It was accordingly an object of the present invention to provide a polymer which contains at least one recurring unit which has electron-transport properties, thus simplifies electron transport in electronic devices, preferably PLEDs, and thereby increases their efficiency.
The object according to the invention has been achieved in accordance with the invention by the provision of a polymer which contains at least one structural unit of the following formula (I):

where the symbols and indices used have the following meaning:
X is selected from

Y1 and Y2 are each selected, independently of one another, from C(R1)2, NR1, C(R1)2—C(R1)2, CR1═CR1 and a single bond, but with the proviso that either Y1 or Y2 is a single bond,
m is 0 or 1,
R1 is in each case selected, independently of one another, from H, D, F, Cl, Br, I, N(Ar1)2, C(═O)Ar1, P(═O)Ar12, S(═O)Ar1, S(═O)2Ar1, CR2═CR2Ar1, CN, NO2, Si(R2)3, B(OR2)2, OSO2R2, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms, a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, each of which may be substituted by one or more radicals R2, where one or more non-adjacent CH2 groups may be replaced by R2C═CR2, C≡C, Si(R2)2, Ge(R2)2, Sn(R2)2, C═O, C═S, C═Se, C═NR2, P(═O)(R2), SO, SO2, NR2, O, S or CONR2 and where one or more H atoms may be replaced by F, Cl, Br, I, CN or NO2, an aryl, aryloxy, heteroaryl or heteroaryloxy group having 5 to 40 C atoms, which may also be substituted by one or more non-aromatic radicals R1, where, in addition, two or more radicals, preferably two adjacent radicals, R1 may form an aliphatic or aromatic, mono- or polycyclic ring system with one another, or an electron-transport unit R3, but with the proviso that at least one of the radicals R1 is an electron-transport unit R3,
Ar1 is selected on each occurrence, in each case independently of one another, from an aryl or heteroaryl group or an aromatic or heteroaromatic ring system,
R2 is in each case, independently of one another, H, an aliphatic hydrocarbon radical having 1 to 20 C atoms or an aromatic hydrocarbon radical having 6 to 20 C atoms, where two or more radicals R2 may also form a ring system with one another,
R3 is an electron-transport unit,
n is in each case, independently of one another, 1, 2, 3 or 4, and
where the dashed lines on the two outer phenyl rings represent the bonds to the adjacent structural units in the polymer.